You can add tasks without a due date by simply leaving out a date when talking to Siri (for iOS 11+ and watchOS 4+):

Add "Read Jane Goodall's book" to my task list in RTM

Second, if Siri responds by asking, "When would you like me to remind you?" you can say:

Whenever

For earlier versions of iOS and watchOS, or if you are using the CalDAV integration:

You can add tasks without a due date in a few different ways:

First, you can simply leave out a date when requesting to Siri:

Remind me to read Jane Goodall's book

Second, if Siri responds by asking, "When would you like me to remind you?" you can say:

Whenever

Last, you can also add the task in another way:

Add "Read Jane Goodall's book" to my Remember The Milk list.